undo.sh -- bash

$ cat git/undo.md

[ GIT ] — undo for version control

# How do I undo the last commit but keep my changes staged?

$ git reset --soft HEAD~1

→ Moves HEAD back one commit. Index and working tree untouched.
→ Your changes remain staged, ready to recommit.

# Discard ALL local changes and reset to remote

$ git reset --hard origin/main

⚠ DESTRUCTIVE — uncommitted work is lost. There is no "trash".

# Undo a commit that's already pushed (preserves history)

$ git revert <sha>

→ Creates a new commit that inverts <sha>.
→ Safe for shared branches: rewrites nothing.

# Throw away changes to a single file

$ git restore path/to/file.txt

→ Replaces the working-tree copy with the indexed version.

# I accidentally dropped a stash — can I get it back?

$ git fsck --no-reflog | awk '/dangling commit/ {print $3}'

→ Lists dangling commits. Try git show <sha> on each.
→ Recover with git stash apply <sha>.

# I force-pushed and lost my branch — help

$ git reflog | grep 'feature-x'

→ The reflog remembers every HEAD movement for ~90 days.
→ git reset --hard <sha> restores the branch tip.

$ cat files/undo.md

[ FILES ] — undo for the filesystem

# I just rm'd the wrong file. Can I recover it?

$ extundelete /dev/sdaX --restore-file path/to/file

⚠ Stop writing to the disk NOW. Unmount and clone before recovery.

# Undo the last mv

$ mv new/location original/location

→ There is no built-in mv-undo. Reverse the arguments manually.
→ Tip: alias mv='mv -i' to confirm overwrites.

# I overwrote a file with cp

$ ls -la *~

→ If cp -b was used, a backup with ~ suffix exists.
→ Otherwise: check editor swap files (.swp, #file#).

# Restore a file's modification time after editing

$ touch -d '2026-04-30 12:00' file.txt

→ Sets mtime back to the given timestamp.

$ cat permissions/undo.md

[ PERMISSIONS ] — undo for chmod / chown

# I ran chmod -R 777 on /etc — how do I fix it?

$ sudo dpkg --verify | awk '/^.M/'

⚠ Reinstall affected packages: apt --reinstall install <pkg>

# Undo a recursive chown

$ find /path -newer /tmp/marker -exec chown orig:orig {} +

→ Use a marker file's mtime as the boundary.

# Restore the default umask in the current shell

$ umask 022

→ 022 is the canonical user-default. 077 for paranoid mode.

$ cat network/undo.md

[ NETWORK ] — undo for routing & firewall

# I locked myself out with iptables — flush everything

$ sudo iptables -F && iptables -P INPUT ACCEPT

⚠ Schedule with at now + 5 minutes before testing rules remotely.

# Restore the default route after a misconfigured ip route

$ sudo ip route add default via 192.168.1.1

→ Replace the gateway with your network's actual address.

# Remove a stale SSH host key

$ ssh-keygen -R hostname.example.com

→ Surgically removes one host from ~/.ssh/known_hosts.

$ cat processes/undo.md

[ PROCESSES ] — undo for signals & jobs

# I sent SIGSTOP to the wrong PID. Resume it.

$ kill -CONT <pid>

→ SIGCONT undoes a SIGSTOP/SIGTSTP pause.

# Reattach a backgrounded job to the current shell

$ fg %1

→ jobs lists job IDs. Use %n to target one.

# Forgot nohup on a long-running process

$ disown -h %1

→ Removes the job from the shell's HUP table without stopping it.

$ cat packages/undo.md

[ PACKAGES ] — undo for installs & upgrades

# Undo the last apt install

$ sudo apt remove --purge <package>

→ --purge also removes config files in /etc.

# Roll back a brew upgrade

$ brew switch <formula> <old-version>

→ Older versions live in $(brew --cellar).